As glass for a data storage medium substrate such as a magnetic disk or an optical disk, for example, lithium-containing aluminosilicate glass having a high Young's modulus or one having chemical reinforcing treatment applied thereto (e.g. Patent Document 1), or crystallized glass having a crystal layer precipitated by heat-treating glass having a specific composition (e.g. Patent Document 2), is used.
In recent years, along with an increase of the storage capacity of a hard disk drive, development for high recording density has been in progress at a high pace. However, along with the development for high recording density, microsizing of magnetic particles tends to impair the thermal stability, and crosstalk or a decrease in the SN ratio of playback signals is problematic. Here, a heat-assisted magnetic recording technology has attracted attention as a united technique of light and magnetism. This is a technique wherein a magnetic recording layer is irradiated with a laser beam or near-field light to have the coercive force locally reduced at the heated portion, and in such a state, an external magnetic field is applied for recording, and the recorded magnetization is read out by e.g. a GMR element. Thus, recording can be made on a high-coercive force medium, whereby it becomes possible to microsize the magnetic particles while maintaining the thermal stability. However, in order to form such a high-coercive force medium in the form of a multilayer film, the substrate is required to be sufficiently heated, and a highly heat resistant substrate is required.    Patent Document 1: JP-A-2001-180969    Patent Document 2: JP-A-2000-119042